The present invention relates to a new and improved antenna and more particularly, to an omnidirectional, horizontally polarized probe-fed slot antenna having a conductive coupling ring for efficient coupling of RF energy within the antenna.
Slot antennas are constructed by cutting one or more slots in the wall of a radio frequency ("RF") transmission line to provide an efficient transfer of RF energy between each slot and the transmission line, thereby providing a good return loss for the antenna. Such antennas are well known in the art, and are discussed in Silver, S., Microwave Antenna Theory and Design, Boston Technical Publications, pages 305-309 and pages 325-328, and Johnson, Robert C. and Jasik, Henry, Antenna Engineering Handbook, 2nd Edition, McGraw Hill, Chapter 9.
Normally, efficient transfer of RF energy in a slot antenna is accomplished when the placement of the slot interrupts RF currents on the wall of the transmission line. When certain antenna characteristics are desired that constrain a slot to a position where no interruption occurs, transfer of RF energy may be accomplished by the use of a conductive probe adjacent to the slot and penetrating into the transmission line. Such a design is useful, for example, in an antenna where an omnidirectional, horizontally polarized radiation pattern is desired. When such an antenna is used in the transmitting mode, the conductive probe "feeds" RF energy from the transmission line to the slot which then radiates the energy into the external environment. Thus, such devices are often referred to as probe-fed slot antennas.
It has been found that even with conductive probes adjacent to the slots of a slot antenna, inefficient coupling of RF energy can result between the transmission line and the slots resulting in a less than optimal return loss, e.g., a return loss no greater than about 3 dB for any probe depth. The literature describes probe-fed slot antenna designs employing multiple (cascaded) sets of slots in order to overcome the poor coupling which can otherwise result. Efficient coupling between the transmission line and the slots exists in such designs because each set of slots only has to couple a portion of the total coupled energy.
There is often a need for antennas, such as beacon antennas or antennas used with active decoys which, due to size and antenna pattern constraints, cannot be provided with additional sets of cascaded slots to improve the RF energy coupling. It would therefore be advantageous to provide a probe-fed slot antenna having improved coupling of RF energy between the transmission line and slots without resorting to multiple sets of slots. Such an antenna should provide an improved return loss, for example on the order of 15 dB or greater at the antenna's operating frequency. The present invention provides a probe-fed slot antenna meeting these requirements.